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Noise Figure of an upconverting mixer under input IF signal

bennys

I'm trying to simulate the noise figure of a transmit mixer as a function of input IF power. 

From the cadence tutorials it seems as if the HBnoise sim is performed under small signal conditions where the input IF port is set to DC

(for example in http://www.cadence.com/blogs/rf/archive/2011/03/10/tips-for-simulating-a-transmit-mixer-in-spectrerf.aspx, its stated "If you are running PAC or Pnoise, you set the IF sourcetype to dc so that the LO is the only sinusoid in the system (Edit Properties on your IF port in the schematic). Since the input for this example is sinusoidal, we will choose Harmonic Balance. " )

How whould you recommend to perform a noise figure sim with IF signal applied.

 

In addition when I look at the noise separation print form it shows that the noise of the mixer comes from a biasing circuit which is AC disconnected from the circuit..., this must be an artifact and can not be the real noise source of the mixer, not sure but maybe you have some input on this as well? 

Parents
  • bennys said:

    I would also expect that the NF not be dependant on IF input power, but this is not the result I get from simulation (the NF makes sence at low input level but then gets unrealisticly high as the input power is raised).

    Ok, but how are you simulating it then? Maybe it is an artifact of the simulation?

    bennys said:
     

    There is an interest in verifying at different IF power level as you need the Tx to have good linearity at high output power as well as low noise, you therefore need to verify your design when the IF power is set to some setback level from P1dB (depending on required linearity).

    Understood. I would however mantain that unless you drive your IF port into compression (which you will draw from linearity analysis), you should still be 'small signal' for that port, meaning that you should not see appreciable differences in the NF. If you see different results, it may be IMVHO that either you are not simulating correctly, or the linear range of the Mixer is much smaller than you think. 

    bennys said:
     

    As for the noise contribution, I agree that DC noise can be up converted by the mixer but I don't think this is the case here for two reasons:

    1. the noise depends heavaly and is very sensitive to the size of the bias device (a very small change in size may change the NF by 10dB) so it does not make any sense

    2. I ac block the bias circuit by attaching it to a separated power supply and placing a 1Farad shunt capacitor at its output with no effect on noise contribution or actual noise... 

    I think we should first take a look at how you are simulating the circuit. It may be that the simulator is not used properly for the purpose, so it is giving you results that do not make much sense.

    M 

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  • bennys said:

    I would also expect that the NF not be dependant on IF input power, but this is not the result I get from simulation (the NF makes sence at low input level but then gets unrealisticly high as the input power is raised).

    Ok, but how are you simulating it then? Maybe it is an artifact of the simulation?

    bennys said:
     

    There is an interest in verifying at different IF power level as you need the Tx to have good linearity at high output power as well as low noise, you therefore need to verify your design when the IF power is set to some setback level from P1dB (depending on required linearity).

    Understood. I would however mantain that unless you drive your IF port into compression (which you will draw from linearity analysis), you should still be 'small signal' for that port, meaning that you should not see appreciable differences in the NF. If you see different results, it may be IMVHO that either you are not simulating correctly, or the linear range of the Mixer is much smaller than you think. 

    bennys said:
     

    As for the noise contribution, I agree that DC noise can be up converted by the mixer but I don't think this is the case here for two reasons:

    1. the noise depends heavaly and is very sensitive to the size of the bias device (a very small change in size may change the NF by 10dB) so it does not make any sense

    2. I ac block the bias circuit by attaching it to a separated power supply and placing a 1Farad shunt capacitor at its output with no effect on noise contribution or actual noise... 

    I think we should first take a look at how you are simulating the circuit. It may be that the simulator is not used properly for the purpose, so it is giving you results that do not make much sense.

    M 

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